The invention relates to making a nozzle or a nozzle divergent section as a single piece of composite material comprising fiber reinforcement densified by a matrix.
The field of application of the invention is more particularly that of nozzles for rocket engines or aeroengines.
For parts that are to be used in the fields of space or aviation, it is well known to use thermostructural composite materials, i.e. composite materials that have mechanical properties making them suitable for constituting structural elements and also having the ability to conserve these properties at high temperatures. Such thermostructural materials are in particular carbon/carbon (C/C) composite materials (carbon fiber reinforcement and carbon matrix); and ceramic matrix composite materials (CMC), e.g. C/SiC (carbon fiber reinforcement and silicon carbide matrix), C/C—SiC (carbon fiber reinforcement and mixed carbon and silicon carbide matrix), or indeed SiC/SiC.
The fiber reinforcement for thermostructural composite materials may be obtained by winding filaments or by superposing fiber plies on a former so as to obtain a fiber preform having a shape that is close to the shape of the part that is to be made. Fiber plies can be bonded to one another, in particular by needling, using barbed needles that move fibers transversely relative to the plies, thereby providing bonding between the plies so as to increase resistance to delamination, i.e. resistance to the plies separating from one another.
Densifying the fiber reinforcement by a carbon or a ceramic matrix may be performed using a liquid technique or by chemical vapor infiltration (CVI). Densification using a liquid technique comprises, in well-known manner, impregnating the fiber reinforcement with a liquid composition containing a carbon- or ceramic-precursor resin, and then polymerizing and pyrolyzing the resin so as to obtain a carbon or ceramic residue, with it being possible to perform a plurality of consecutive cycles of impregnation, polymerization, and pyrolysis. CVI densification is performed in well-known manner by placing the fiber reinforcement in an enclosure and by admitting a reaction gas into the enclosure such that under determined conditions of pressure and temperature, in particular, the gas diffuses into the fiber reinforcement and serves to deposit matrix material by means of one or more of the components of the gas decomposing or by means of one or more of its components reacting. For parts of particular shape, in particular of complex shape, a first step of consolidation using the liquid technique may be performed on appropriate tooling in order to freeze the fiber reinforcement into the desired shape, with densification being continued without using tooling, e.g. by CVI. Associating consolidation using a liquid technique and densification by CVI is described in particular in document EP-A-0 633 233.
Proposals have already been made to make a nozzle divergent section out of thermostructural composite material.
Thus, document U.S. Pat. No. 6,817,184 discloses a method of fabricating a thin-walled C/SiC material divergent section by winding filaments of carbon fiber yarns and shaping so as to obtain integrated flange portions with continuity of the fiber reinforcement. Carbon fabric inserts may be inserted to increase thickness locally. In that document, a prior technique is mentioned that consists in placing layers of fabric that are superposed on a former, in impregnating the layers of fabric with a carbon precursor resin, and after pyrolyzing the resin, in performing infiltration with molten silicon so as to obtain a C/SiC composite.
Another known process that is used by the Applicant comprises forming fiber reinforcement by superposing and needling fiber plies on a former and densifying the fiber reinforcement by a matrix obtained by CVI. As mentioned above, compared with fiber reinforcement made up of superposed layers that are not bonded to one another, needling provides resistance to delamination, and thus better mechanical strength. In order to obtain needled reinforcement having substantially uniform characteristics, it is nevertheless necessary to make a needled preform that is relatively thick, with only a central portion thereof being usefully usable. For example, in order to obtain useful reinforcement thickness of 3 millimeters (mm), it is necessary to make a needled fiber preform having a total thickness of 20 mm. Making the fiber reinforcement is thus lengthy and expensive, requiring various manipulations, and leading to large losses of material. In addition, the fiber volume percentage in the needled fiber reinforcement is relatively low, thereby limiting the mechanical properties of the resulting nozzle or nozzle divergent section.